Despite the considerable mass appeal of popular diet books, such approaches lack data to support their efficacy and safety. Despite its widespread use for more than 30 years, the Atkins diet has never been evaluated in a large randomized controlled trial (RCT). Data from large RCTs are necessary to help physicians, patients and the broader health care community make informed decisions about the safety and efficacy of these programs. The current proposal will assess the short-term and long-term clinical effects of the Atkins diet in 360 obese men and women. Participants will be randomized to the Atkins diet (low-carbohydrate, unlimited fat and protein) or a conventional USDA diet (high-carbohydrate, low-fat). The effects of each dietary approach on changes in: 1) weight and body composition; 2) metabolic and organ function; and 3) exercise tolerance will be evaluated.